When a pregnant mammal is engaged in the labor and delivery process for her fetus, a common practice is to monitor both the heart rate of the fetus and the uterine tone of the pregnant mammal. The uterine tone of the pregnant mammal provides information regarding the uterine contractions of the pregnant mammal by measuring the pressure exerted by the uterine muscle in units of pressure, for example, millimeters of mercury (mmHg) and/or kilo Pascals (kPg). One way to provide information regarding the fetal heartbeat and uterine tone to a doctor or other healthcare provider is to provide a graph, either in paper or electronic form, that displays a fetal heart rate over time and uterine tone over time. In most cases, this information is synchronized so that the fetal heartbeat and uterine tone for a particular moment in time may be simultaneously observed. By comparing the fetal heart rate at a particular moment in time with the uterine tone at that same moment in time, a doctor may be able to determine whether the fetal heart rate decreases when the pregnant mammal experiences a contraction.
FIGS. 1A and 1B provide two examples of simultaneously displayed fetal heartbeat and uterine tone for corresponding moments in time. In FIGS. 1A and 1B, graphs 10A and 10B, respectively, display fetal heartbeat in beats per minute as a function of time where each vertical line provided on the grid represents one minute. In FIGS. 1A and 1B, graphs 12A and 12B, respectively, display uterine tone in mmHg and kPa as a function of time. In FIG. 1A, graph 10A shows fetal heart rate within a normal range of 120-180 beats per minute and there are no obvious fluctuations in the fetal heart rate that correspond with changes in uterine tone. With the information provided by FIG. 1A, a doctor may draw the conclusion that the fetus is not being negatively impacted by the uterine contractions and is not in distress. In contrast, graph 10B shows a fetal heart rate that experiences significant dips (e.g., from approximately 150 beats per minute prior to a uterine contraction to below 90 beats per minute during an immediately following a uterine contraction) that correspond with uterine contractions (i.e., increases in pressure within the uterus). With the information provided by FIG. 1B, a doctor may draw the conclusion that the fetus is being negatively impacted by the uterine contractions and may be in distress (e.g., experiencing a lack of oxygen that may cause neurologic damage). Upon drawing this conclusion, the doctor may decide that the fetus' health is in danger and, therefore, it should be surgically removed from the uterus via a Caesarian section (C-section). However, a change in fetal heart rate of the type shown in FIG. 1B does not always indicate that the fetus is in distress as there are many other possible causes for a drop in fetal heart rate. Thus, the doctor may prescribe a C-section when one is not needed causing undue harm to the pregnant mammal.
Oximetry is a method for determining the oxygen saturation of hemoglobin in a mammal's blood. Typically, 90% (or higher) of an adult human's hemoglobin is saturated with (i.e., bonded to) oxygen while only 30-60% of a fetus's blood is saturated with oxygen.
Pulse oximetry is a type of oximetry that uses changes in arterial blood volume through a heart beat cycle to internally calibrate oxygen saturation measurements of the oxygen level of the blood.
Current methods of performing fetal oximetry are flawed for many reasons. For example, while U.S. Patent Publication No. 2004/0116789 describes a fetal oximeter using pulse oximetry, this oximeter is flawed for at least three reasons. First, the wavelengths of the electro-magnetic radiation used by the '789 Publication to determine fetal oximetry are short and consequently cannot travel a distance through the abdomen of the pregnant mammal so as to reach the fetus with sufficient strength. Thus, the signal reflected signal is too weak to decipher. Second, the '789 Publication is flawed because of the assumptions included therein are based on research with adult hemoglobin, which is fundamentally different from fetal hemoglobin because fetal hemoglobin has a different structure than adult hemoglobin and therefore absorbs/reflects light differently. Finally, the '789 Application does not process the received signal to reduce noise.
Like the '789 Publication, Patent WO 2009032168 describes a fetal oximeter using near-infrared spectroscopy but fails to provide a signal processing algorithm. In addition, the WO 2009032168 uses assumptions regarding adult hemoglobin to determine fetal oximetry, which yields inaccurate results because, as noted above, fetal hemoglobin and adult hemoglobin have different structures and, therefore reflect light differently.
U.S. Patent Publication No. 2011/0218413 describes an algorithm for signal processing that uses maternal electrocardiography (ECG), Doppler, and pulse oximetry. However, for at least the reasons pointed out above, trying to obtain a fetal oximetry signal using maternal (i.e., adult) pulse oximetry won't work. Furthermore, the '413 Publication fails to make any compensation for structural differences in fetal and adult hemoglobin.
U.S. Patent Publication No. 2011/0218413 provides another example wherein a pregnant mammal wears a belt that shines light towards the belly and fetus that is detected on the other side of the abdomen. The distance traveled by the light would be 15-30 inches, or 35 to 75 cm, and this is not technically feasible because the signal received by the detector would be too weak to decipher. The light looses intensity quickly and there are FDA limitations on how intense the light directed into a pregnant mammal's abdomen can be because light that is too intense could cause, for example, burns to the pregnant mammal and retinal damage to the fetus.